Filter



June 27, 1944. F. W YOUNG FILTER Filed Aug. 3, 1940 2 Sheets-Sheet l wi mw mi l lla tbbbs Fmg/wf w. y0u/v6: BY ATTORNEY.

June 27, 1944. F w, YQUNG 2,352,304

FILTER Filed Aug. 3, 1940 zrsneets-sheet 2 I NV EN TOR.

ATTORNEY.

Patented June 27, 1944 UNITED STATES PATENTA g OFFICE 6 Claims. l

This invention relates to methods and apparatus for washing solids recovered from liquids in which they have been suspended and particularly to solids of a chemical nature which renders recovery difficult due to foaming. for instance, of the filtrate.

In the washing of brown stock (caustic sulphate wood pulp) in kraft mills, it is necessary at the present time, after the wood has `been digested in the caustic liquid, to wash the resulting pulp free of the caustic liquor, resin and other impurities that have been disassociated by the digesting process. For reasons of economy, it is of advantage to recover the major portion of the caustic liquor for reuse. This has heretofore been done by washing the pulp in diffusera or wash pans and has required large and expensive equipment and considerable labor because of the large quantity of foaming liquid present. More recently, the pulp has been washed on continuous filters by countercurrent washing using, for instance, two filters for two stage countercurrent washing, three filters for three stage countercurrent washing, and so on. The latter method of washing reduces labor and decreases the loss of caustic but the handling and rehandling of thev pulp and vliquors in each washing stage causes considerable and very troublesome foam.V Because of this foam, the pumps, tanks and other equipment r in the system as a whole are vented into a plurality of foam 'tanks or traps, many of these being needed and much space being occupied.

One object of the present invention is to performA multi-stage countercurrent washing of "Brown Stock on a single filter.

Another object of the invention is to avoid repulping between washing stages.

A further object of the invention is to reduce `to a minimum the amount of foam produced and .ing returned to the filter for use in cake discharge.

It is a further object of the invention to utilize all the-gaseous medium withdrawn from the filter in the filtering operation by returningit to the filter for cake discharge land during the 1 course of its return under pressure to utilize it.

`The invention also seeks a method, and apparatus for carrying out the method, which is practical from the standpoint of ease and cheapness -of installation and practicabiiity in use. These andother objects of the invention and vthe apparatus of this invention;

Figure 2 is a view, on an enlarged scale, showing a filter by which'multi-stage washing in accordance with the process of this invention may vbe performed; and

Figure 3 is a fragmentary view showing the upper end of the barometric leg and the connection to the vacuum pump.

Referring first to Figure 2, the continuous filter of this invention is shown as comprising a filter drum III rotatably mounted on an axially extending cylindrical bearing member l2. This cylindrical bearing member I2 is shown as taking the form of a tube.

The filter drum l0 is shown as formed by a pair of spaced discs 22 centrally apertured,las at 24, to receive the cylindrical bearing I2. 'The circumferential surface of the drum is formed by a cylindrical member 38-40 which may be composed of more than one part or as an integral cylinder and is formed with a plurality of closely spaced filtrate passages 40 dened by Wall portions 38. Filtering medium 9 overlies the surface of the drum and is supported by the shell 38.

The filter drum rotates in the filter tank 46, Figure 1, through which the cylindrical bearing ymember I2 passes and the filter tank 46 is open to the atmosphere. The opening 41 through which the trunnion I2 passes to the outside of the tank is sealed.

'Ihe filter may be rotated in any .convenient manner.

Inwardly of the filter drum, the cylindrical bearing is formed with a plurality of filtrate pas'- sages 54 whereby fluid froml the interior of the filter drum may enter the cylindrical bearing for evacuation from the drum. The total area of passages 54 are greater than the internal crosssectional area of bearing tube I2 so that filtrate from the interior of the drum has unobstructed access to tube i2 in volume up to the carrying vcapacity of tube I2. During operation, the minimum level of the liquid inside the drum will never be lower than the lowest passage 54 because the vpassages 54 in the wall liquid flows out by gravity. As is well known, during operation as a vacuum filter, the level oi' the liquid inside the drum will not be higher Vthan the highest passage I4 because, in order to maintain vacuum in the drum, air must be evacuated through tube I2 and cannot be so evacuated should all outlets be submerged. T he structure here described is shown, described and claimed in my copending application Serial Number 291,083 filed August 19, 1939.

It is proposed, in accordance with one aspect of the invention, to remove the filtered material from the surface of the filter drum by a pressure differential upon opposite faces thereof. Specifically, it is proposed to force fluid i. e., gaseous medium such as air, from the drum by a vacuum pump outwardly through the filtration passages 4U, formed in the drum periphery. To this end, a conduit, such as the pipe 66, adapted to conduct air or other fluid medium under pressure, is disposed axially of the cylindrical bearing I2 and an elbow or bent extremity l2 on the end thereof terminates within the drum interior.

In the illustrated embodiment, a shoe or box receives the gaseous medium under pressure from this conduit 64, 68. As shown, the outer wall 62 (Figure 2) of the shoe is provided with an aperture 64 of a size to register with and be preferably coextensive with one of the passages 40 in the drum periphery. The inner box wall 66 opposite the aperture 84 is formed with a tubular passage 68 to receive the fluid from the conduit end 58.

The filter drum is applicable lfor use in either high or low submergence operation. Proper operation of the filter of this invention is dependent upon internal submergence of the lower end of th'e box or shield 6| in the liquid or filtrate in the drum. Figure 2 shows a filter, in which, the level of the filtrate II in the drum is below the center line of the filter and the shield 6I, |06 extends from ahead of cake discharge 64 to apoint beyond the liquid level |I.

As the drum rotates, filter cake on the surface thereof is lifted of! the surface by the pressure of the fluid passing through the apertures 64 and 40 so that it is separated from the filtering medium on the drum and carried away by the doctor blade Il.

To wash the filter cake, nozzles 86, 61, 88 may be mounted, in known manner, in spaced relation to the filter medium on the filter drum I0. The wash liquid falling, say, from a nozzle 66 ahead of the point where the cake leaves the drum, passes through the filter cake on thati'segment of the drum periphery and enters a pan 90 mounted therebeneath. An outlet pipe 92 from the pan 90 passes through one of the filtrate I of the cylindrical bearing member I2 and from thence is conducted to pump 91. The weak wash liquor may then be pumped as by pump 81 throughfpipe 96 to nozzles 21 and pass through the filter cake into pan 98 from whence itis drawn through outlet pipe which stronger wash liquor may be pumped as by the pump IUI through pipe |03 to nozzles 88. The wash liquor from nozzles I8 passes through the filter cake and into the interior of the drum where it mixes with the filtrate and is evacuated by pipe I2. This description is of a three stage countercurren't washing and to those versed in the art is known `to be regularly accomplished by using three filters. Due to the novel construction of this filter, unlimited stages of countercurrent Washing may be accomplished on a single filter. 75

through the instrumentality of the vacuum' pump |24.

A barometric leg is used, as at I l5, for evacuating the liquid. Some air drawn through the filter medium is carried down with the liquid in the barometric leg and into the closedY tank III, where the air and liquid separate, the liquid flowing out of the closed tank III through the pipe f I 20 which traps the air under pressure in the closed seal tank IIB and controls the pressure of air according to the height of the outlet |22 controlling the level II9 tank III.

The air is accumulated in the upper part. of tank III at the is allowed free flow through pipe lines |22 into the blow-back pipe 56 leading to the air box 6|-68. Thus the volume of air and the pressure thereof produced in the closed tank III can be utilized in the air box 6I-66 to discharge the filter cake.

The air travelling upwardly in the pipe |22 carries with it relatively small quantities of foam. This foam interferes materially with theoperation of the filter and accumulation of filtrate for xeuse. To reduce or destroy this foam and prevent the entranceA of the foam into the blow-back pipe 56, gaseous medium under pressure is introduced into the pipe |22 at a suitable point between the seal tank I i2 and the filter or pipe B4 therein. This gaseous medium is conveniently ob. tained from the filter drum by a vacuum pump and used in a closed cycle. As shown in Figure 3, the tube I2 at one side of the filter drum |l terminates in a T fitting, from one end of which the barometric leg |05 extends. From the upper end of the T fitting, a conduit I 22 leads tcV a vacuum pump |24 which constantly draws nuid (air) evacuated from the drum I6 and forcee it through a conduit |26 -back to the pipe |22. Preferably this air is forced into the pipe |22 in a downward direction and to this end the pipe |22 is shown as provided with a Y fitting |22 to the upwardly extending branch |29 of which the conduit is connected. The air under pressure entering the pipe |22 at this point disrupts the bubbles of foam in the air forced up from the tank II8 and causesthe liquid of the bubbles to fall back down the pipe |22 into the seal tank whereby substantially only air may rise above the fitting |28 to enter the bIow-backpipe 66.

'I'he caustic sulphate wood pulp is deliveredv into the blow tank I3I where it settles to the bottom and is agitated by the agitator |23 driven by motor |25. Overflow pulp from the filter tank 48 is also conducted to the blow tank ISI by conduit |31. Caustic liquor from liquor tank |22, which has been delivered thereto from seal tank IIB is pumped by pump I4I through conduit |42 to the blow tank I3.I to dilute the pulp therein. Liquor is also delivered by pump I4I through conduit |45 to the filter tank 46 to further dilute the pulp and through a branch conduitv |41 and conduit |23 as sealing liquor for the pump. The caustic pulp partially diluted is withdrawn through conduit |48 from the blow tank by pump 0! the liquid in thepressure produced therein and l I| and delivered to the filter tank 46. The various percentages on the drawings show the dilution of the pulp. The indications of the strength of the caustic liquor are indicated on the drawings at the different points in degrees Baume.

The entire system illustrated in Figure l is hermetically sealed whereby no outside air may leave the system except through the filter medium at the point of cake discharge. The air enters the filter drum during the washing and dewatering period. This air is sucked out by the vacuum pump. The discharge of the vacuum pump is returned to the system and used for discharging the filter cake or pulp sheet. Some air may be entrained rwith the liquid taken down the barometric seal leg. This is liberated in the seal tank and returns with the other blow-back air. Some air may be entrained with the liquid taken out by pumps y91 and lOl. the filter with the wash liquor and liberated from the Wash sprays `86, 81 and 88. The liquor for reuse in filtration and that sent to the evaporator is collected, free of air, in the closed liquor tank |39. The small amount of foam (bubbles) that may form in the seal tank is knocked down at |28 by the pressure from the discharge of the vacuum pump so that no foam, tanks or traps are required.

Various modifications will occur to those skilled in the art in the composition, configuration and disposition of thecomponent elements going to make up the invention as a whole as well as in the use of selected elements in given situations and no limitation is intended by the phraseology of the foregoing description or illustrations in the accompanying drawings except as indicated in the appended claims.

What is claimed is:

1. The method of preventing the accumulation of foam on the surface of the liquid evacuated from a filter which comprises subjecting the liquid and gas to pressure produced -by introducing al1 the gaseous medium removed from the interior of the filter into an enclosure and returning al1 the gaseous medium through said enclosure to the filter for use in cake discharge.

2. In a device of the character described, in combination, a rotary drum filter, means to selectively deliver washing fluid to predetermined areas of filter cake adjacent the external surface of the drum, means to selectively receive and return the washing fluid from one said area to a different said area, means to separately evacuate liquid and gaseous medium from the drum interior, means to further separate the gaseous medium under pressure from the fluid and return said separated gaseous medium under pressure to a predetermined area of that srface of the filter cake adjacent the external drum surface to discharge the filter cake therefrom and means to seal said last mentioned predetermined area-on the interior of the drum surface against the subatmospheric pressure in the drum to permit the said discharge of the said filter cake.

3. In a device of the character described, in

'I'his is returned to combination, a rotary drum filter, means to selectively deliver washing fluid to the external drum surface, means within the drum to receive fluid from the respective spray nozzles, conduits to evacuate liquid from the respective pans to the respective nozzles, a barometric leg to evacuate liquid and gaseous medium from the drum interior, means to further separate the gaseous me-` dium under pressure from the fluid, means to return said separated gaseous medium to a predetermined external area of that surface of the filter cake adjacent the drum surface to discharge the filter cake therefrom comprising a barometric leg, a tank into which the barometric leg enters and which s sealed against the escape of gaseous medium, a acuum pump in communieating connection with the barometric leg at a point proximate the filter and means to deliver gaseous medium from the pump to said means returning the gaseous medium returning means, and means to seal said last mentioned predetermined area on the interior of the drum surface against the subatmospheric pressure in the drum tokpermit the said discharge of the said filter ca e.

4. In a multi-stage washing of brown stock, in combination, a continuous filter, barometric means to create subatmospheric pressure in the filter, gaseous pressure cake discharge means. means to separate gaseous medium and the foam created thereby from the barometric means, means to deliver said separated gaseous medium and entrained foam to the cake discharge means,

means to conduct gaseous medium from the lbarometric means to the last named means to break down the entrained foam and thereby to prevent foam returning to the filter.

5. In the multi-stage counter-current washing of brown stock, in combination, a rotary drum filter, a pump to evacuate gaseous medium and the foam created thereby from the filter, a barometric leg evacuating liquid and entrained gaseous medium from the drum interior, a closed tank into which the barometric leg empties and in which the gaseous medium separates from the liquid and is placed under pressure, a conduit to deliver the gaseous medium under pressure from the tank to the inner surface of the filter cake on the filter drum to discharge cake therefrom, a conduit connecting the pump with the upper portion of the barometric leg and a conduit leading from the pump to the first named conduit to deliver fluid evacuated from the drum to the gaseous medium delivery conduit to break down foam rising in the conduit from the closed tank therein.

6. The method of preventing the return of foam to a filter which comprises returning all of the gaseous medium removed from the interior of the iilter to the interior of the filter for use in cake discharge and during such return, utilizing a portion of the gaseous medium during its return to the interior of the filter to break down foam in the returning gaseous medium. Y

FRANK W. YGUNG. 

